An Afghan policeman destroys a poppy field in Kunar province: just 14 of Afghanistan’s 34 provinces are 'poppy free'. Photograph: Rahmat Gul/AP

Twelve years after the fall of the Taliban, Afghanistan is heading for a near-record opium crop as instability pushes up the amount of land planted with illegal but lucrative poppies, according to a bleak UN report.

The rapid growth of poppy farming as western troops head home reflects particularly badly on Britain, which was designated "lead nation" for counter-narcotics work over a decade ago.

"Poppy cultivation is not only expected to expand in areas where it already existed in 2012 … but also in new areas or areas where poppy cultivation was stopped," the Afghanistan Opium Winter Risk Assessment found.

The growth in opium cultivation reflects both spreading instability and concerns about the future. Farmers are more likely to plant the deadly crop in areas of high violence or where they have not received any agricultural aid, the report said.

Opium traders are often happy to provide seeds, fertilisers and even advance payments to encourage crops, leaving farmers who do not have western or government agricultural help very vulnerable to their inducements.

At the same time the more powerful figures in the drugs trade, from traffickers to corrupt government officials, who take over half the profit from each kilo of opium, have shrinking opportunities to earn money from Nato or international aid contracts – and may be preparing a war chest for upcoming presidential and parliamentary elections.

"Opium cultivation is up for the third successive year, and production is heading towards record levels," said Jean-Luc Lemahieu, Afghanistan head of the UN Office on Drugs and Crime. "People are hedging against an insecure future both politically and economically."

Just 14 of Afghanistan's 34 provinces are now "poppy free", down from 20 in 2010. In three provinces, the spring sowing was the first time this decade that farmers had risked an attempt at growing opium.

The only figures showing a fall in cultivation, for western Herat province, may actually be due to a statistics blip. The UN was forced to use external data last year instead of the satellite images that are usually the basis of poppy growing calculations, and local officials protested heavily that the opium crop there had been overestimated.

If this year's poppy fields are harvested without disruption, the country would likely regain its status as producer of 90% of the world's opium. Afghanistan's share of the deadly market slipped to around 75% after bad weather and a blight slashed production over the past two years.

But the decline in opium production also drove up prices, to a record $300 a kilogramme. Prices have now slipped by over $100 but are still far above historic levels, helping tempt more farmers to turn land over to poppy.

It seems unlikely that the poor harvests of the last year will be repeated; there have been no reports of blight and the exceptionally bitter winter of 2011-12 was followed this year by a milder one, creating expectations of a large crop.

The increase has come despite a marked improvement in Afghanistan's specialised counter-narcotics units, Lemahieu said. Fear of eradication has become a far more significant reason for farmers to stick to legal crops than in the past, the report found.

But overall the government and aid community has not prioritised efforts to cut back a crop and trade that feeds global markets for heroin, Lemahieu said, despite its corrosive effect on security, corruption and trust in Kabul.

Typical of the official neglect are the 22 "national priority programmes" drawn up by Kabul to focus aid money and diplomatic efforts on its key development concerns including justice and education. Counter-narcotics was not one of them, nor has it been put at the heart of the other programmes.

"We need to have counter-narcotics dealt with seriously by the entire government as well as the aid community," Lemahieu said. "One of the big missing links here is providing for the communities themselves."

Eradication programmes that do not provide farmers with benefits such as healthcare and education, and support growing other crops will just push the Taliban or other insurgent groups that do tolerate or encourage poppy production, he added.